The present invention relates to the prediction of the time when a mobile station will depart a cell of a satellite mobile telephone system.
Known satellite mobile telephone systems include the Inmarsat-M system, the IRIDIUM(trademark) system described in, for example EP-A-0365885, the ICO(trademark) system described in, for example, GB-A-2295296 and the ODYSSEY(trademark) system described in, for example, EP-A-0510789. Whilst each of these systems is cellular in nature, they differ from terrestrial systems in that the cells move relative to the Earth""s surface because each cell is defined by a beam from an orbiting satellite.
To extend battery life during the idle mode of a satellite mobile telephone, the mobile telephone is desirably powered down for 95% to 98% of the time. Periodically, the mobile telephone wakes up briefly to determine if broadcast control channels from satellites can be received. If so the mobile telephone checks for an incoming call. In idle mode, the mobile telephone needs to know when to hand over to another beam from the same or a different satellite, i.e. when to start listening to another broadcast control channel.
If a mobile telephone does not know the time of the next handover or the new broadcast control channel, it must frequently search a number of frequencies, for example 8. In particular, if on wake-up the broadcast control channel is found to be weak or absent, the mobile telephone has no way of knowing whether the signal is being blocked or interfered with or whether it is now being serviced by another beam.
It is desirable for a mobile telephone to check only two or three broadcast control channel frequencies (one from a primary satellite and one or two from secondary satellites) instead of all possible frequencies. This minimises xe2x80x9con timexe2x80x9d during each wake-up while maintaining the desired low duty cycle. Frequent wake-ups are desirable to minimise the time taken to detect incoming calls and the return of signals after an outage.
It is an aim of the present invention to overcome this problem.
Briefly stated, in a system according to the present invention, a mobile station predicts when it will move into another cell on the basis of one set of broadcast information thereby reducing the overall processing burden.
According to the present invention, there is provided a method of operation of a satellite mobile telephone system in which a plurality of cells move across the surface of the Earth as a satellite orbits, the method comprising the steps of: providing a mobile station with information related to the movement of a cell relative to the Earth""s surface; and determining at the mobile station a prediction for the time when the mobile station will leave said cell on the basis of said received information. The method may involve providing the information to the mobile station by broadcasting the information from a satellite to a cell, the information defining the geographical position of the cell, and receiving said information for the cell at the mobile station.
According to the present invention, there is also provided satellite mobile telephone system comprising transmitting means for transmitting control data in a control channel, wherein the control data comprises geographical information defining the position of a cell associated with the control channel.
According to the present invention, there is further provided a mobile telephone for a satellite mobile telephone system, including memory means for storing control data including geographical information related to the movement of a cell across the Earth""s surface and processing means for processing said information to make a prediction of when the mobile station will depart the current cell. The mobile station may include receiving means for receiving said geographical data, wherein said data relates to the cell in which the mobile station is located.
Although the mobile station must perform some numerical calculations, these are not burdensome compared with repeated scanning of all broadcast control channel frequencies.
If the cells differ in size or shape, the information preferably includes information regarding the extent of the cell. However, the mobile station may include information regarding the size and shape of each cell and, in this case, the information need only include the identity of the cell in which the mobile station is located. The designer of a system according to the present invention will be expected to make a design choice balancing the conflicting requirements of transmission capacity and mobile station complexity.
If the shape of the cell is unchanging, or the mobile station stores information regarding the shapes of cells, the information will include information regarding the traversing of the cell over the Earth""s surface. In a system in which the cells rotate relative to the Earth""s surface, the information will preferably include information regarding rotational movement of the cell.
If the mobile station is provided in advance with no geographical information regarding the cell, the information preferably comprises latitude and longitude co-ordinate values for the centre of the cell, a vector defining the traversing of the cell over the surface of the Earth, co-ordinate values for the vertices of the cell relative to the centre of the cell, at least one vector value defining rotation of the cell as it traverses the surface of the Earth and a time value.
Preferably, the information comprises a set of translational vectors defining the traversing of the cell over the surface of the Earth during predetermined sub-periods of a period during which said information remains unchanged. The information may similarly comprise a set of vectors defining the rotation of the cell. If the sub-periods are relatively short, it may be sufficient for the mobile station merely to identify the sub-period during which it will leave the cell. However, it is preferred that the mobile station derive a particular time, either by identifying a point within one of the sub-periods or directly from the received information.
As a by product of the prediction of the cell departure time, the mobile station will determine a prediction for the cell that it will be entering. Accordingly, the mobile station can identify the control channels that it should monitor after departing a cell.
Since a network will often be aware of a mobile station""s location and the movement of cells across the Earth, the mobile station preferably only need re-register with the network if its prediction is not correct, indicating that the mobile station has moved. Accordingly, there is preferably a step of determining at the mobile station the correctness of the prediction and performing a network registration process for the mobile station in dependence on said determined correctness.